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Thread: Does the Thumb Hole weaken the steel?

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    Spyderco Forum Registered User SpyderEdgeForever's Avatar
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    Does the Thumb Hole weaken the steel?

    I ask because of former discussions on stress points, and also, there seem to be differing viewpoints on the web. Someone claimed they had a knife with a thumbstud, and when subjected to extreme stress, the blade broke at the stud point. But the Spyder hole is rounded, and the circular shape is one of if not the strongest in nature, and I tend to think it does not weaken it. I would appreciate other views and feedback.

  2. #2
    Spyderco Forum Registered User yo mama's Avatar
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    Thought is maybe horizontally, but not vertically.

    In the end, I've never seen a blade break there, usually it's tips that are chipped, or edges rolled.

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    source: google images
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    Spyderco Forum Registered User wrdwrght's Avatar
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    I've asked myself this question as well and have concluded that while the blade is certainly weakened, the hole (or resulting thin strip of steel forming its upper reach) would not be the first point of failure. The force that could cause this failure would first cause the pivot to fail, which is merely to admit that prying with a knife (perish the thought) or batoning with it should be left to fixed blades and only thick ones at that. In other words, within the slicing and carving parameters for which it was designed, a Spydie's weakened blade only matters conceptually. I'd sure like Sal to confirm or deny this surmise.

    ETA: The foregoing picture does suggest my surmise may be faulty...but I see its source to be Google Images so we can't know what caused this failure. Anyone on the Forum have such a failure?
    Last edited by wrdwrght; 12-09-2013 at 04:36 PM.
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    Spyderco Forum Registered User The Deacon's Avatar
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    Weaken the steel, probably not. Weaken the blade, probably. I suspect that, like most other things in life, the Spyderhole is a trade off. I'm certain the blade would be stronger without it but, to me at least, the benefit of the ease of opening the Spyderhole provides far outweighs the increased risk of the blade snapping if I decide to stick it between two railroad ties horizontally and stand on the handle, or do something equally stupid and/or intentionally destructive.
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    Spyderco Forum Registered User Evil D's Avatar
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    I can break anything if I try and/or abuse it hard enough. Use it as a knife to cut things and it'll never break. Use it as a pry bar and you get what you deserve.

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  7. #7
    Spyderco Forum Registered User Blerv's Avatar
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    How many Spyderco blades are broken at the hole compared to those in circulation being thrashed?
    Blake

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    http://www.handgunlaw.us/documents/USKnife.pdf

  8. #8
    Spyderco Forum Registered User FCM415's Avatar
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    It probably does weaken the blade. I will never break one in my lifetime though, I can't imagine a task that I do that would put that much stress in any of my Spydercos. If I ever do, I'll resurrect this thread and tell you about it.

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    Quote Originally Posted by tap78 View Post
    source: google images
    Two can play at that game



    The point I'm trying to make is that blades can break at every point, so finding anecdotes about breaks at the thumb hole doesn't mean much in terms of the actual design. What I will say, though, is that if you are going to have material missing in the blade, a round hole is the way to go. This is due to something called stress concentration: essentially, a round edge distributes the stress along many different points, whereas something like a notch or square edge will result in higher concentrations of that stress. In fact, failure to take this into account caused two planes to crash in 1954, killing 56 people. This is why airplanes do not have square windows like your house does, and instead use windows with rounded corners.

    Without actually running stress tests myself, I'd be very, very surprised if you could induce material failure along the strong axis of the blade by simply leaning into a cut. Instead, I'd assume - and very strongly so - that there was some torsion involved, and when the knife was twisted, it failed. Either that or it was used for prying, which again is along the weak axis. In fact, the picture I just posted was originally posted on these forums by spoonrobot, whose brother broke the knife by prying something. And let's not overlook that the knife failed nowhere near the thumb hole, showing us that the blade could fail this way with or without the thumbhole.

    http://en.wikipedia.org/wiki/Stress_...tor_for_cracks

    http://en.wikipedia.org/wiki/De_Havi...asters_of_1954

    http://www.spyderco.com/forums/showt...roken-Delica-4

  10. #10
    Spyderco Forum Registered User xceptnl's Avatar
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    I would bet it is less a matter of stress points, and more a physics related mater related to the cross sectional area that the point of highest stress. The maximum allowable moment for any stressed member is going to occur near the bearing point unless a weaker cross section is present near to the fulcrum point. If stresses had anything to do with it, I would examine the removed areas in relation to the time of heat treatment.
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  11. #11
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    Quote Originally Posted by xceptnl View Post
    I would bet it is less a matter of stress points, and more a physics related mater related to the cross sectional area that the point of highest stress. The maximum allowable moment for any stressed member is going to occur near the bearing point unless a weaker cross section is present near to the fulcrum point. If stresses had anything to do with it, I would examine the removed areas in relation to the time of heat treatment.
    Moments cause stress, stress leads to failure

    http://en.wikipedia.org/wiki/Euler-B...quation#Stress

  12. #12
    Spyderco Forum Registered User xceptnl's Avatar
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    Quote Originally Posted by Hayes View Post
    Moments cause stress, stress leads to failure

    http://en.wikipedia.org/wiki/Euler-B...quation#Stress
    I was speaking the the materials stresses that can occur from drilling, punching, grinding of a material. Not the stress caused from the Moment force.
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  13. #13
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    Quote Originally Posted by xceptnl View Post
    I was speaking the the materials stresses that can occur from drilling, punching, grinding of a material. Not the stress caused from the Moment force.
    I believe OP was specifically wondering whether the thumb hole - as in the absence of material - makes the blade weaker under applied force rather than the material itself coming weaker from the manufacturing process:

    Quote Originally Posted by SpyderEdgeForever View Post
    I ask because of former discussions on stress points, and also, there seem to be differing viewpoints on the web. Someone claimed they had a knife with a thumbstud, and when subjected to extreme stress, the blade broke at the stud point. But the Spyder hole is rounded, and the circular shape is one of if not the strongest in nature, and I tend to think it does not weaken it. I would appreciate other views and feedback.
    But both are valid things to take into consideration when discussing material failure, sure.

  14. #14
    Spyderco Forum Registered User yablanowitz's Avatar
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    Quote Originally Posted by Hayes View Post
    I believe OP was specifically wondering whether the thumb hole - as in the absence of material - makes the blade weaker under applied force rather than the material itself coming weaker from the manufacturing process.
    I believe you're correct, but that isn't what he actually asked.

    Does the Spyderhole weaken the steel? No. The steel is the same. Does the absence of steel where the Spyderhole is make the blade weaker? Yes. Air is much weaker than steel. Is the Spyderhole likely to result in a decrease in service life of the blade? Probably not, if the blade is used as a cutting tool. Is it likely to decrease the life of the blade if it is used as a crowbar? Probably. Tools are not universally interchangeable. I remember the story posted with the images above, and lateral loading was definitely involved. The moral of the story is: If you want to pry things apart, use a prying tool, not a cutting tool. The owner of that Military had not yet learned that lesson, and probably never will.
    I don't believe in safe queens, only in pre-need replacements.

  15. #15
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    Quote Originally Posted by xceptnl View Post
    I was speaking the the materials stresses that can occur from drilling, punching, grinding of a material. Not the stress caused from the Moment force.
    One of the benefits of cutting the Spyderhole with a laser is negligible stresses in the adjacent material. High end circular saw bodies are laser cut for this same reason.

  16. #16
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    Hi Yo Mama, Hayes,

    Welcome to the Spyderco forum.

    sal

  17. #17
    Spyderco Forum Registered User xceptnl's Avatar
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    Quote Originally Posted by Bill1170 View Post
    One of the benefits of cutting the Spyderhole with a laser is negligible stresses in the adjacent material. High end circular saw bodies are laser cut for this same reason.
    So the only real additional stresses could be from the threading the of thumb-stud models and this would likely be minimal correct.
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  18. #18
    Spyderco Forum Registered User
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    Quote Originally Posted by sal View Post
    Hi Yo Mama, Hayes,

    Welcome to the Spyderco forum.

    sal
    Thanks, Sal! I've been lurking for a while, this forum's been a great source of information.

    Quote Originally Posted by yablanowitz View Post
    I believe you're correct, but that isn't what he actually asked.
    My mistake then. I had read it as "does the having a hole in the steel make the blade weaker", to which my answer was "yes, but to nowhere near the level at which it becomes a risk of failure under normal use".

  19. #19
    Spyderco Forum Registered User
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    If the hole for a thumb stud is lasered, then threaded, there are still more stress risers due to threads being sharp. But I'm thinking primarily of Spyderholes which are smooth inside, and large.

  20. #20
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    I suspect the blade hole is ultimately machined since they frequently have a very sharp edge. Laser cutting will leave more residual stresses around the edge of a hole than machining will, but I don't think either is a factor since the blades are heat treated after the holes are cut.

    Threads in a thumb stud hole would not be a stress riser since the threads would be parallel to the direction of stress. The hole itself would cause a stress concentration though.

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